If I click on Edit by an existing comment, change one of the CCK fields, and press save, I get this error:

Duplicate entry '0' for key 2 query: UPDATE issuetracker_node SET vid = 0, type = 'issue', language = '', title = 'Secure FTP on sw2web and s31wpf', uid = 1, status = 1, created = 1244650912, changed = 1259620655, comment = 2, promote = 0, moderate = 0, sticky = 0, tnid = 0, translate = 0 WHERE nid = 69

(This comes from Krumo and devel module.)